The present invention relates to the manufacture of glass and more particularly to a glass melting furnace and its operation.
In a known method of manufacturing glass in a continuous process, raw materials are fed in at one end of a glass melting tank to form a blanket floating on an existing bath of molten glass. The rate of feeding is sufficient to maintain a constant glass depth in the tank whilst molten glass progressively flows towards the opposite end of the tank known as the working end, from which molten glass is taken away for use in a forming process. The blanket of raw materials is converted to molten glass as it passes through a melting zone at one end of the tank by heat which may come for example from burning fuel supplied from burners situated at spaced intervals in the side walls above the glass level or from electrical heating devices. The molten glass passes from the melting zone into a refining zone where heat is also supplied above the molten glass. In the refining zone bubbles of gas still remaining in the glass are encouraged to escape or go into solution in the glass. The glass passes from the refining zone into a conditioning zone adjacent the working end of the tank. In the conditioning zone the glass is homogenised and brought to a suitable thermal condition for use in the forming process. Normally a canal leads from the working end of the tank to a forming process.
From the above, it can be seen that certain regions of the tank are defined as melting, refining and conditioning zones. As regards the molten glass passing from one zone to another, all the glass leaving any one zone may not necessarily have reached a final state for that operation e.g. a fully refined state as it enters the conditioning zone. Some refining can still occur in the conditioning zone, and conditioning may start to some extent in the refining region. Hence the zoned regions are defined to show the areas in which the greater part of all of a particular operation is carried out in a tank, and enables the man practised in the art to identify the temperature conditions required in these zones.
Conventionally heat is supplied for melting and refining the glass by the combustion of liquid or gaseous fuel above the glass surface, by electric heating within the body of the glass or by a combination of both methods; the glass in the conditioning zone is normally cooled by air blown across the free surface of the glass.
A rising temperature gradient is arranged along the melting zone of the furnace by control of the energy input along the furnace length, the temperature reaching a maximum at a so called hot spot; downstream of this position the temperature is caused to fall. The effect of these temperature gradients is to cause convection currents which return hot glass in the upper layer of the melting zone underneath the batch blanket towards the filling end, so augmenting the heat supplied to the main body of glass in the melting zone which would otherwise not be heated sufficiently as the unmelted batch forms an insulating layer which interferes with the transfer of heat to the main body of molten glass beneath. The temperature gradients also cause, downstream of the hot spot, convective flows which carry glass in the upper layers of the refining zone forwards towards the conditioning zone, returning colder glass in the lower layers of the refining zone back towards the hot spot. These convective flows serve to homogenise the glass and the colder lower layers of glass prevent furnace bottom refractories reaching a temperature sufficiently high for rapid chemical attack and erosion.
Melting, refining and conditioning are all time and temperature dependent; maximum temperatures being limited by the ability of the furnace refractories to withstand these temperatures, and the time spent by the glass in any particular zone being limited by the furnace geometry. Thus for any particular design of furnace there is a maximum output above which deterioration in glass quality will arise.
Even when operating a tank within its designated limits, it is sometimes difficult to obtain completely homogeneous glass free from undissolved solids and gases and uniform in composition; the problem becomes greater as the output of the tank is increased. Glass, varying in composition, forms layers in the tank, these layers being subject to convective and other flows imposed by the furnace operation, design and other physical operations carried out on the glass. In general, in the final product, the layers are parallel to the glass surface but there may be deviation from this parallel state in areas subjected to a change in flow conditions, e.g. in the centre region of a ribbon of glass. Where the layers cease to be parallel to the faces of the glass, optical faults occur.
Various means are available for improving this situation, for example, improving the thermal efficiency by insulating the furnace structure, using improved refractory materials to reduce corrosion and erosion, changing glass composition so that less heat is required to melt and refine the glass, or changing methods of supplying heat to the glass to improve the effectiveness of this heat. However, it is generally found that additional output from a furnace cannot be achieved without increased cost, reduced furnace life or a deterioration in glass properties.